1. Field of the Invention
This invention relates to a dot matrix display panel used for a large-sized character or pattern display panel for a portable liquid crystal television or office automation system, and more particularly to a dot matrix display panel with a thin film transistor array and the manufacturing method for the panel.
2. Description of the Prior Art
In order to display liquid crystal or the like in a matrix by a low duty factor, a switching element comprising a thin film transistor (to be hereinafter abbreviated to TFT) has hitherto been tried to be introduced into each picture element. In detail, a substrate 10 with a TFT array, as shown in FIG. 1 plan view and FIG. 2 sectional view on the line A-A', is constituted of a gate electrode 1, a gate insulating film 2, a semiconductor layer 3, a drain electrode 4, a source electrode 5 and a picture element electrode 8 connected to the drain electrode 4 through a contact hole 9, each at a picture element unit on an insulating substrate 7 such as glass. A display medium 12, such as a liquid crystal, as shown in FIG. 3 sectional view, is sandwiched between tne substrate 10 with the TFT array and a transparent insulating substrate 14, such as glass, having thereon a transparent common electrode 13 such as indium oxide or tin oxide, whereby a X-Y matrix display panel capable of displaying a number of picture elements can be constituted.
Referring to FIG. 4, an electrical equivalent circuit of the matrix display panel with TFT array shown in FIGS. 1 to 3, is shown.
Next, explanation will be given on the principle of operation of the panel with reference to FIG. 4.
For convenience, the TFT is assumed to be of an n-channel enhancement type using CdSe or amorphous silicon as the semiconductor layer. In this case, in the state where a voltage is applied to make the drain electrode 4 positive with respect to the source electrode 5, when the gate electrode 1 is kept at a potential equal to or less than that of the source electrode 5, the TFT is off and a current scarcely flows between the source and the drain, but when the gate electrode 1 is kept positive with respect to the source electrode 5, electrons are induced within the semiconductor layer 3 in contact with the gate insulating film 2, resulting in the TFT being on so that a current flows between the source and the drain. For matrix-driving, signals are usually provided in the order of the line. In other words, at the time of simultaneously applying to signal lines X.sub.1, X.sub.2, X.sub.3. . . corresponding to the source electrodes 5 an on-signal (i.e.--a positive voltage with respect to that of the transparent common electrode 13) or an off-signal (i.e.--an equal or lower potential with respect to that of the electrode 13), a selection pulse (which is positive with respect to the transparent common electrode 13) is applied to one of scan electrodes Y.sub.1, Y.sub.2 . . . corresponding to the gate electrodes 1. The nonselected scan electrodes are kept at a nonselected potential (which is equal to or less than that of the transparent common electrode 13). Among the picture elements connected to the signal line to which the selection pulse is applied, the picture element supplied with the on-signal supplies a voltage to the display medium 12 because an electrical capacitor comprising the picture element electrode 8, transparent common electrode 13 and display medium 12 between the electrodes 9 and 13, is charged. On the contrary, the picture element supplied with the off-signal supplies no voltage to the display medium 12. In this way, the TFT element operates as a switch to prevent crosstalk. The charge disappears in accordance with a time constant depending on an off-resistance of the TFT, a resistance of the picture element and the capacitance of the picture element. For the display medium, such as a liquid crystal, which deteriorates during its life span unless driven by an AC voltage, a field sequential AC voltage must be supplied to the picture element.
In the aforesaid description, since the display panel mainly uses a twisted nematic liquid crystal as the display medium 12, when the picture element electrode 8 and the gate insulating film 2 or the semiconductor film 3 provided on the picture element electrode 8 are provided, these films 2 and 3 are restricted so as to be transparent. For example, in a case of using an amorphous silicon formed by the plasma CVD method, since the film is opaque, the process of removing the opaque semiconductor film on the picture element electrode 8 has been required. However, in a case where the gate insulating film 2 is of silicon nitride or silicon dioxide and the semiconductor layer 3 is of amorphous silicon or polysilicon, it is difficult to carry out the selective etching for removing the amorphous silicon or polysilicon semiconductor without damaging the gate insulating film 2 or the transparent picture element electrode 8, whereby strict control of the etching process has been required to obtain a thin film transistor having a superior performance. Even if the process is carefully controlled, it is actually difficult to produce the TFT array with a high yield.
When the TFT array in FIGS. 1 and 2 is manufactured by use of, for example, the plasma CVD method, the process comprising the following steps is necessary:
1. A step of forming a transparent electrode, such as tin oxide or indium oxide, on the glass substrate 7.
2. A step of using a first photomask to pattern the transparent electrode so that the transparent electrode is patterned by the photoetching into the form of the picture element electrode 8 in FIG. 1.
3. A step of forming a gate electrode material, such as chromium.
4. a step of using a second photomask to pattern the gate electrode material into the form of the gate and scan electrode 1 in FIG. 1.
5. A step of depositing the gate insulating film 2, such as silicon nitride or silicon dioxide, by use of the plasma CVD method.
6. A step of depositing an amorphous silicon semiconductor film by use of the plasma CVD method.
7. A step of using a third photomask to pattern the amorphous silicon semiconductor film in the vicinity of the picture element into the form as shown by reference 3 in FIG. 1 and to simultaneously etching-remove the amorphous silicon semiconductor film at the terminal pick-up portion at the peripheral portion of the display panel.
8. A step of using a fourth photomask to etching-remove the gate insulating film at the peripheral portion of the panel and the contact hole 9 in FIG. 1 between the drain electrode 4 and the picture element electrode 8.
9. A step of forming an electrode material for the source and drain electrodes 5 and 4.
10. A step of using a fifth photomask to pattern the electrode material into the form of the source electrode 5 and the drain electrode 4 in FIG. 1.
Thus, the transparent picture electrode 8 is covered by the transparent dielectric film for the gate insulating film 2 to constitute the TFT array. In a case where the dielectric film 2 is larger in thickness because it covers the picture element electrode 8, the film causes a voltage drop to raise the drive voltage, whereby the dielectric film 2 on the picture element should be etching-removed by the use of a sixth photomask. In the step 8, the simultaneous removal of the dielectric film 2 on the picture element has been considered, but, if the transparent electrode is exposed, the transparent picture element electrode 8, in the step 10, leads to its exposure to an aluminum etching liquid when using, for example, aluminum or the like as the material for the source and drain electrodes 5 and 4, thereby damaging the transparent picture element electrode 8. Hence, in the step 8, the gate insulating film 2 need remain on the picture element electrode 8, as the protective film therefor in the subsequent steps.
The abovementioned conventional TFT array formation process requires at least five to six photomasks so as to be troublesome, thereby having mainly caused the lowering of the yield and the raising of the manufacturing cost.
As seen from FIG. 1, the regions of the source electrode 5 and the gate electrode 1 cannot be displayed, but only the region of picture element electrode 8 except for the contact portion can substantially contribute to the display. In other words, the source electrode 5 and the gate electrode 1 significantly reduce the effective area of the picture element, and even when the density thereof is high, the FIG. 1 pattern is not similarly reducible, because the source and gate electrodes, when smaller in width, increase wiring resistances to easily cause distortion of signal waveform or a wire disconnection. This leads to a further lowering of the effective area thereof, thus having created a serious problem in that the construction of electrodes in FIG. 1, especially for a high resolution display, lowers the display quality so as to lead to a macroscopic lowering of contrast.